Thermal Siphon Solar heating.

[Crickleymal]

Just a quick question here. Supposing one were to make an indirect thermal siphon water heating set up but for the sake of simplicity and cheapness feed the supply from the header tank to the flow side rather than the return.

Normally the supply goes to the return side and an expansion/overflow pipe goes to the header tank. So if you were to do it the way I suggest how much heat would be lost to the header tank?

[Zech]

You mean like this?


Quite a lot, I'd think. I can't see why the hot water would continue past the junction to your cylinder much at all - the header tank would (should) be higher, and therefore a more attractive route.

I'm a bit puzzled by your use of the word "supply". In an indirect system presumably you're not actually supplying fresh water all the time, but circulating the same body of water round and round, with a header tank to allow for expansion. This probably means I've misunderstood you.

[Odsox]

I'm not so sure Rachel, although I can't quite make up my mind.
If as you say the header tank would be a more attractive route, it still has to get a return supply from somewhere. The only "somewhere" available is through the cylinder coil which would mean a thermo siphon flow which would in turn give up heat to the cylinder.
Either it would work (but not as efficiently as it could) or it won't work at all and there would be no flow, the header tank would get hot due to conduction and the cylinder would stay relatively cold.
But I can't make up my mind which ... sorry Malc.

[Zech]

I see what you mean - it might still draw the water round the system and so work. Even so, unless the header tank was much better insulated than they usually are (which is, of course, an option) a lot of heat would be lost that way.

[Crickleymal]

You mean like this?

That image doesn't show up either in IE or Firefox here at work. and if I type in the address mandraulically it shows a blank page. Probably settings on our network.

I'm a bit puzzled by your use of the word "supply". In an indirect system presumably you're not actually supplying fresh water all the time, but circulating the same body of water round and round, with a header tank to allow for expansion. This probably means I've misunderstood you.

You have to supply the indirect coil with water from somewhere otherwise it would be empty and wouldn't work. Basically yes the header tank for expansion is the supply to the indirect coil.

[Zech]

You mean like this?

That image doesn't show up either in IE or Firefox here at work. and if I type in the address mandraulically it shows a blank page. Probably settings on our network.

Oh, sorry about that. Probably my fault for trying to use facebook to host the image. It wasn't a very good drawing anyway - one pipe running uphill from solar panel to coil in cylinder, with a junction on that pipe and a vertical pipe leading up to the expansion tank, then another pipe running back downhill from coil to panel. The key point is that the pipe to the header tank comes off between the panel and the coil.

I'm a bit puzzled by your use of the word "supply". In an indirect system presumably you're not actually supplying fresh water all the time, but circulating the same body of water round and round, with a header tank to allow for expansion. This probably means I've misunderstood you.

You have to supply the indirect coil with water from somewhere otherwise it would be empty and wouldn't work. Basically yes the header tank for expansion is the supply to the indirect coil.

Yes, I did misunderstand - I took 'supply' to mean continuously supplying more fresh water (as in replacing water that comes out of the taps), which wasn't what you meant.

[Crickleymal]

Thanks. I thought it would be less efficient but I was trying to save a bit of copper